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What is the characteristic impedance of a quarter wave transformer matching 200 ohms to 50 ohms?

  • A.250 ohms, the sum of the two impedances that are to be matched together
  • B.100 ohms
  • C.125 ohms, the arithmetic mean of the two impedances being matched
  • D.150 ohms

Answer: B

The transformer impedance is the geometric mean of the two:

\[ Z = \sqrt{200 \times 50} = 100 \]

so 100 ohms. The arithmetic mean is the natural wrong answer and is worth guarding against. On a typical substrate a hundred ohm line is narrow but perfectly printable, which is part of why the technique is so common. The geometric mean rule applies to any quarter wave transformer, in any medium, and is worth remembering in that general form.

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